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CT Flashcards

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1
Q

CT Artefact - Beam hardening

A

Beam hardening artefacts appear as streaks and shadows adjacent to areas of high density such as the petrous bone, shoulders, and hips The artefact occurs because the high density anatomy absorbs the lower energy photons while the higher energy photons pass through to the detectors which results in the beam becoming ‘harder’ Iterative reconstruction and bowtie filter reduces beam hardening artefact

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2
Q

CT Artefact - Partial Voluming

A

Tissues of widely different absorption are encompassed on the same CT voxel producing a beam attenuation proportional to the average value of these tissues Thicker slices and smaller matrix is more prone to partial voluming Mitigated by thinner slices / smaller pixels - however image will appear much noisier unless there is a compensatory increase in the mAs or reduction in pitch

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3
Q

CT Artefact - Ring Artefact

A

Faulty detector element

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4
Q

CT - Noise

A

mAs: reduced mAs will decrease the photon flux density and will increase noise kV: decreasing kV will increase contrast, but less photon flux at detector, thereby increasing noise Matrix size / pixel size: reduced pixel size will increase spatial resolution but will decrease photon flux per pixel resulting in increased noise Slice thickness: the thicker the slice, the higher the photon flux in each voxel, reducing noise but reducing spatial resolution Pitch: decreasing the pitch will increase overlapping of slices and increase the photon flux per pixel, reducing noise Patient size: thicker/larger patients will lead to reduced photon penetration (reduced photon flux on the image receptor) leading to increased noise increased chance of scatter in thicker patients reduces the contrast Algorithm: bone algorithm increase noise, but improves spatial resolution (as opposed to soft tissue algorithm which improves contrast)

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5
Q

CT - Spatial resolution

A

Matrix size: smaller matrix size Slice thickness: thinner slices Sharp algorithm: image appears noisier, but better spatial resolution

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6
Q

CT - Contrast

A

Ionic markedly hypertonic to achieve sufficient concentration of iodine high osmolality and high viscosity -> 4 fold increase in side effects (vascular, cardiac) vs non-ionic Non-ionic low osmolality Side effects mild = nausea, vomiting, warmth moderate = rash, vasovagal, bronchospasm, laryngeal oedema, vasovagal severe = severe bronchospasm, severe laryngeal oedema, cardiac arrest, anaphylaxis risk of death = 1/170,000 extravasation can result in erythema to tissue necrosis contrast induced nephropathy = acute renal failure within 48 hrs of contrast administration high chance of CIN in at risk patients (eg. diabetes, CKD) therefore patients should be well hydrated Iodinated contrast can cross placenta -> effects on foetus not well documented

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7
Q

CT Fluoroscopy

A

Display of continuously updated images produced by continuous rotation of a CT tube Generally performed at same kV but lower mA than conventional CT (120kV, 50mA as opposed to 200-300mA) System requirements: Slip rings for continuous scanning Fast tube rotation (1 second) High heat capacity tube to allow for extended scanning times Fast image reconstruction (> 3 images per second) Cine image display allowing last image hold and video recording Bed mounted control / foot switch Rapid image reconstruction: Each image not a complete reconstruction each image uses some new data, but mostly data from previous images eg. tube spins 60 degrees per image, therefore this data added to the other 300 degrees from previous images and the previous 60 degrees is subtracted reconstruction is still via back-projection Usually reconstructed on a 256×256 matrix, rather than the standard 512×512 matrix Dose to patient and operator: Tube current is low to reduce dose but it is concentrated to a smaller area than conventional CT (therefore deterministic effects can occur eg. skin erythema) For patients, the overall dose is comparable to conventional CT For the operator considerable doses can be accumulated, especially to the skin of the hands * Ways to reduce operator dose ensure that the hands are out of the beam when the scanner is operating use of a pair of forceps to manipulate the biopsy or drainage needle from a distance

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8
Q

CT Fluoroscopy Radiation Risk

A

Risks to the doctor performing the procedure include: effect from radiation exposure, especially on the limb (hand/arm) used to insert the needle/drain within the x-ray beam especially during continuous image acquistion stochastic (cancer induction) and deterministic effects (skin erythema, necrosis) needle stick injury from biopsy/drainage needle Precautions to minimize risks of radiation exposure:* Time: minimise scan times to reduce radiation dose screen intermittently and only when required (as opposed to continuous) make use of previous acquired images to review anatomy (last image hold) Distance: Stand as far away from the x-ray source as possible, though procedural work will require the doctor standing next to the patient the needle/drain etc should be held in needle holders or forceps to avoid having the doctors hands within the x-ray beam * Shielding: wear lead aprons, boots, calf shields, thyroid shield, lead glasses, moveable lead shields, lead gloves if possible Technique: if possible, CT guided procedure can be performed by intermittent scanning planning scan performed marking performed by radiographer on patients skin needle insert, doctor exits the CT room and pt is scanned in region of interest to check position doctor re-enters room and repositions needle etc and exits again to rescan and check position allows the doctor to avoid radiation exposure altogether, though is more time consuming than CT fluoroscopy

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9
Q

Parameters affecting patient dose

A
  1. kVp
  2. mAs
  3. tube current modulation
  4. beam shaping filter
  5. pitch
  6. scan length
  7. field of view
  8. algorithm
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10
Q

Factors affecting patient dose: Patient size

A
  • Higher mAs required for obese patients
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11
Q

Factors affecting patient dose: Rotation Time

A
  • Faster rotation = decreased dose through lower expsosure
  • Increased noise
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12
Q

Factors affecting patient dose: Beam Quality

A
  • Higher kVp = more photons produced per mAs = higher dose (by power of 2)
  • Higher kVp = decreased subject contrast
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13
Q

Factors affecting patient dose: Scanned Volume

A
  • Decrease the scanned volume to only the clinical area of interest
  • Scan patient in one large block rather than two or more smaller blocks
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14
Q

Factors affecting patient dose:Reconstruction algorithm

A
  • Bony algorithm = increased spatial resolution BUT also increased noise

Smoothing algorithm = lower resolution BUT also lower noise

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15
Q

Factors affecting patient dose: Beam Width

A
  • Large collimation reduces the effects of overbeaming as it results in a smaller penumbra
  • Over beaming is increased through
    • Short beam width (short collimation)
    • Large focal spot
    • Poor radiographic geometry e.g. short SDD
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16
Q

Factors affecting patient dose: Multiphase study

A
  • Choose only phases that are relevant e.g. no delayed phase
17
Q

Factors affecting patient dose: Tube current modulation

A
  • Using this feature enables dose reduction in areas of low attenuation e.g. the lung without impact on quality
18
Q

Factors affecting patient dose: Pitch

A
  • Pitch greater than 1 means more movement of the gantry per slice and so reduction in patient exposure
  • Increases noise secondary to partial volume averaging
  • Head CT should be performed in sequential mode with broad collimation to avoid overscanning and overbeaming
19
Q

Factors affecting patient dose: mAs

A
  • Reduction leads to lower dose
  • Due to less photons this also increases noise and hence decreases SNR
20
Q
  • Standard sequential axial CT
A
  • Typically a 3rd generation geometry (rotate/rotate with wide fan beam)
  • Fan beam wide enough to cover whole patient during each slice
  • Single detector which is a scintillator coupled to a photodetector

Slice width determined by pre-patient collimation

21
Q

Helical spiral CT

A
  • 6th generation geometry: data is acquired while patient moving using SLIP RING technology
  • Slice width defined by pre-patient collimation
  • Introduces concept of ‘PITCH’ (typical value P=1-2)
    • Table movement per 360degrees/collimator width (single detector)
    • Table movement per 360degrees/detector width
  • Problem of over-scanning (over-ranging)
    • Helical scanners must irradiate a larger volume of patient than is displayed in order to complete data set – i.e you scan the eye to scan neck
    • Typically an extra rotation at beginning and end of imaged volume

↑ dose in general AND may introduce sensitive organs e.g. gonads

22
Q
  • MDCT
A
  • Helical CT + multiple array of detectors of equal width (7th generation)
  • Can use a CONE BEAM rather than FAN BEAM (single slice)
  • Detector width is determined by binning together N number of detectors
    • Slice width and collimated beam width are DECOUPLED which results in a different definition of PITCH
  • Technical factors improving dose
    • 2 scenarios where dose is reduced
      • By scanning thin slices, one single data set is acquired which can be simultaneously used for either high or normal resolution e.g. HRCT without having to rescan the patient
      • Increased scanning speed due to shorter rotation time and wider beam, ability to cover scan volume within a single breath-hold is improved reduction in motion artefacts and reduced need for repeat examinations
23
Q

Technical factors/problems specific to MDCT which INCREASE dose

A
  • Over-beaming (penumbral effects)
    • Primary collimation needs to be wider than N x slice thickness to avoid penumbral effects (reduced quality) at the outer portions of detector array. This results in increased dose
    • Worst for quad-slice. Improves with increased number of detectors
    • Improves with wider collimation (reduced penumbra)
  • Over scanning (over-ranging)
    • More significant for MDCT (may irradiate a few extra cm)
  • MDCT geometry + detector inefficiencies
    • Limiting spatial resolution in z-direction is dictated by width of discrete detector elements
    • Leads to wasted radiation and loss of geometric efficiency
  • Cone beam
    • Cone beam increases the amount of scatter
    • This is overcome by either
      • Dose increase
      • Technical means associated with a decrease in geometric efficiency
  • Improved radiation utilisation
    • More of the generated x-rays are utilised as it is picked up by the multiple rows
      • Lower tube loading
      • Much more significant dose increase if scanned volume is increased or there are multiple phases
  • Increasing use of thin slice reconstruction
    • MDCT allows same volume to be reconstructed with thin slices without increasing scan time
    • BUT image noise is increased for thinner slices
    • Operator may increase tube loading (mAs) to compensate for increased image noise
24
Q

Factors affecting noise

A
  1. kVp
  2. mAs
  3. pitch
  4. slice width
  5. detector width
  6. smoothing algorithm
  7. matrix size
25
Q

Noise: Slice Width

A
  • Thick slices = better counting statistic and reduced noise
  • Also reduced spatial resolution and increased partial volume artefacts
  • There is not usually an impact on dose unless operator decreases mAs for thicker slices
26
Q

Noise: mAs

A
  • Increased mAs increased total number of photons which reduces noise. Directly proportional to dose
27
Q

Noise: KvP/Beam Quality

A
  • Increased kVp may decrease noise because of significant increase in number of photons. This overcomes the reduction in subject contrast that comes from higher kVP
  • Also contributes to increased dose by approximately a factor of 2
28
Q

Noise: Reconstruction algorithm

A
  • Bony algorithm = increased spatial resolution BUT also increased noise
  • Smoothing algorithm = lower resolution BUT also lower noise
    • Can help reduce dose by switching to less noisy algorithm instead of increasing mAs
29
Q

Noise: Object size and composition

A
  • Large, dense objects (e.g. obese patient) attenuate the beam more severely and reduce number of available photons increased noise

This is overcome by increasing mAs

30
Q

Noise: Matrix size

A
  • Small matrix (64 x 64) will be less noisy but also worse spatial resolution than (256 x 256)
  • No effect on dose
31
Q

Factos affecting spatial resolution

A
  • Detector pitch (center to center spacing of detectors along the array)
  • Detector aperture (width of active element of one detector)
    • Smaller detectors increase the Nyquist Frequency of the image, improving spatial resolution
  • Number of views
    • More views allows CT image to convey higher spatial frequencies without artefacts (aliasing)
  • Focal spot size
    • Larger focal spot = worse geometric unsharpness
    • Becomes more of an issue with greater magnification
  • Magnification
    • Increased magnification amplifies blurring of focal spot
  • Slice thickness
    • Thick slice = poor spatial resolution
  • Pitch
    • Greater pitch = worse resolution
  • Reconstruction kerneal
    • Bone filter = best spatial resolution
    • Smoothing filter = worse spatial resolution
  • Pixel matrix
    • Large matrix (512x512) = better resolution
  • FOV
    • Small FOV allows for better resolution as the size of each pixel is decreased
  • Patient motion
    • Reduces resolution secondary to blur
32
Q
A
33
Q

Artefact: Motion

A
  • High density foreign volume
  • Loss of line integral information leads to streaking e.g. dental implant or metal prosthesis
34
Q

Artefact: Aliasing

A
  • Use of too few projection images acquired to reconstruct high-frequency objects
35
Q

Patient Based Artefact:

A
  • Motion
  • Transient interruption of contrast
36
Q

Physics Based artefact

A
  • Beam hardening
    • Cupping
    • Streak and dark bands
    • Metal artefact
  • Partial volume averaging
  • Quantum mottle
  • Photon starvation
  • Aliasing
37
Q

Hardware Based artefact

A
  • Ring artefact
  • Tube arcing
  • Out of field artefacts
38
Q

Helical and Multichannel aftefact

A
  • Windmill artefact
  • Cone beam effect
  • MPR artefact
    • Zebra artefact
    • Stair step artefact

Radiology Physics (11 decks)

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